sc4508a Semtech Corporation, sc4508a Datasheet - Page 9

• Manufacturer Part Number: sc4508a
• Description: Sc4508a Buck Or Buck-boost Inverting Current-mode Controller
• Manufacturer: Semtech Corporation

When determining the ESR value, both the steady state
ripple-voltage and the dynamic load transient need to be
considered. To keep the steady state output ripple-voltage
< V
To limit the dynamic output voltage overshoot/
undershoot within
voltage) from no load to full load, the ESR value should
satisfy:
Then, the required ESR value of the output capacitors
should be:
The voltage rating of aluminum capacitors should be at
least 1.5V
be greater than:
Usually it is necessary to have several capacitors of the
same type in parallel to satisfy the ESR requirement.
The voltage ripple cause by the capacitor charge/
discharge should be an order of magnitude smaller than
the voltage ripple caused by the ESR. To guarantee this,
the capacitance should satisfy:
Buck-boost converter has higher ripple current than buck
in output. The RMS value is the most important factor to
consider. It has to be less than the output capacitor ripple
current rating.
The buck-boost output capacitor RMS current is:
where, V
In many applications, several low ESR ceramic capacitors
are added in parallel with the aluminum capacitors in
order to further reduce ESR and improve high frequency
decoupling. Because the values of capacitance and ESR
are usually different in ceramic and aluminum capacitors,
Application Information (Cont.)
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2007 Semtech Corp.
o
, the ESR should satisfy:
d
and I
o
. The RMS current ripple rating should also
d
R
are rectifier forward voltage and current.
esr
I
RMS
= min{R
(say 3%) of the steady state output
_
C
CAP
v
R
R
o
ESR
esr
esr
2
2
1
2
esr1
I
I
O
o
3
R
10
f
,R
s
R
esr
I
V
V
V
I
o
o
esr2
o
esr
o
O
V
I
d
IN
}
V
d
9
the following remarks are made to clarify some practical
issues.
Remark 1: High frequency ceramic capacitors may not
carry most of the ripple current. It also depends on the
capacitor value. Only when the capacitor value is set
properly, the effect of ceramic capacitor low ESR starts
to be significant.
For example, if a 10 F, 4m
connected in parallel with 2x1500 F, 90m
capacitors, the ripple current in the ceramic capacitor is
only about 42% of the current in the electrolytic
capacitors at the ripple frequency. If a 100 F, 2m
ceramic capacitor is used, the ripple current in the
ceramic capacitor will be about 4.2 times of that in the
electrolytic capacitors. When two 100 F, 2m
capacitors are used, the current ratio increases to 8.3.
In this case most of the ripple current flows in the
ceramic decoupling capacitor. The ESR of the ceramic
capacitors will then determine the output ripple-voltage.
Remark 2: The total equivalent capacitance of the filter
bank is not simply the sum of all the paralleled capacitors.
The total equivalent ESR is not simply the parallel
combination of all the individual ESRs either. Instead they
should be calculated using the following formulae.
where R
electrolytic capacitors, and R
capacitance of the ceramic capacitors respectively.
(Figure 2)
C
R
eq
eq
(
(
: )
: )
1a
R
R (
a 1
R (
a 1
and C
R
2
a 1
b 1
C
R (
R (
a 1
Figure 2. Equivalent RC branch.
R1a
C1a
R
a 1
a 1
1a
b 1
R
)
b 1
R
2
are the ESR and capacitance of
R
2
b 1
C
b 1
2
)
R1b
C
b 1
)
2
C1b
a 1
)
2
2
2
C
C
2
C
C
a 1
1b
b 1
a 1
a 1
2
2
2
C
and C
C
C
b 1
b 1
b 1
ceramic capacitor is
(
2
C
2
a 1
(
R (
C
1b
(
C
SC4508A
a 1
b 1
C
are the ESR and
a 1
C
www.semtech.com
b 1
b 1
C
)
2
2
C
b 1
b 1
electrolytic
)
R
)
Req
2
Ceq
a 1
ceramic
C
a 1
2
)